1. Field of the Disclosure
This disclosure relates to a flow discharge device and an outlet for a flow discharge device. The disclosure may be concerned with such an outlet for discharging compressor bleed air into a bypass duct of a gas turbine engine.
2. Description of the Related Art
Referring to FIG. 1, a ducted fan (or turbofan) gas turbine engine (e.g. a jet engine) generally indicated at 10 has a principal and rotational axis 11. The engine 10 comprises, in axial flow series, an air intake 12, a propulsive fan 13, an intermediate pressure compressor 14, a high-pressure compressor 15, combustion equipment 16, a high-pressure turbine 17, an intermediate pressure turbine 18, a low-pressure turbine 19 and a core exhaust nozzle 20. A nacelle 21 generally surrounds the engine 10 and defines the intake 12, a bypass duct 22 defined by an inner wall 27 and an outer wall 26, and an exhaust nozzle 23.
The gas turbine engine 10 works in the conventional manner so that air entering the intake 12 is accelerated by the fan 13 to produce two air flows: a first airflow A into the intermediate pressure compressor 14 of the engine core and a second airflow B which passes through the bypass duct 22 to provide propulsive thrust. The intermediate pressure compressor 14 compresses the airflow A directed into it before delivering that air to the high pressure compressor 15 where further compression takes place.
The compressed air exhausted from the high-pressure compressor 15 is directed into the combustion equipment 16 where it is mixed with fuel and the mixture combusted. The resultant hot combustion products then expand through, and thereby drive, the high, intermediate and low pressure turbines 17, 18, 19 before being exhausted through the nozzle 20 to provide additional propulsive thrust. The high, intermediate and low-pressure turbines 17, 18, 19 respectively drive the high and intermediate pressure compressors 15, 14 and the fan 13 by suitable interconnecting shafts.
During engine operation and particularly when changing rotational speed at low power it is important to ensure that the pressure ratio across each compressor 14, 15 remains below a critical working point, otherwise the engine 10 can surge and flow through the engine 10 breaks down. This can cause damage to the engine's components as well as aircraft handling problems.
To maintain a preferred pressure difference across a compressor 14, 15, or even just one stage of a compressor 14, 15, bleed assemblies 30 are provided to release pressure from an upstream part of a compressor 14,15. For example, when a gas turbine engine is operating under transient conditions, e.g. when decelerating, it may be necessary to bleed air at high pressure from the core gas flow A through the engine. Operation of a bleed assembly 30 and engine operability are described in “The Jet Engine” 6th Edition, 2005, Rolls-Royce plc, pages 79-80, and details of such operation will therefore only be briefly mentioned herein.
Referring to FIG. 2, the bleed assemblies 30 may each bleed air through a discharge device 36 into the bypass flow B within the bypass duct 22. The inner wall 27 of the bypass duct (see FIG. 1) is omitted from FIG. 2, so that the compressor casing structure 29 is visible. The bleed assembly 30 comprises a bleed valve 34 which communicates at one end with the respective compressor 14, 15 and is provided at its other end with the discharge device 36. In operation of the engine shown in FIG. 1, part of the core engine air flow A may be diverted through the bleed assembly 30 by opening the bleed valve 34 so that the bleed air flow passes from the respective compressors 14, 15 to be discharged into the bypass duct 22 through the exit 32.
The discharge device 36 may have an open exit 32 that forms a jet flow that penetrates into the bypass flow B. However, such an arrangement is not particularly effective at mixing the hot flow of vent gas with the cool bypass flow, with the result that the hot gas impinges on the downstream surface of the engine casing. This can cause significant thermal damage to the structure unless it is properly protected from the heat, which can increase the weight of the engine as well as the overall cost.
In an alternative arrangement to that shown in FIG. 2, the exit 32 may comprise a so-called pepper pot that has a plurality of openings or holes through which the bleed flow passes into the bypass flow B. However, the individual flow jets from the pepper pot holes tend to coalesce into a single plume, and the bleed flow does not mix well with the main flow.